Plethysmography

How the Test is Performed

You will sit in a large airtight cabin known as a body box. The walls of the cabin are clear so that you and the health care provider can see each other. You will breathe or pant against a mouthpiece. Clips will be put on your nose to close off your nostrils. Depending on the information your doctor is looking for, the mouthpiece may be open at first, and then closed.

You will breathe against the mouthpiece in both the open and closed positions. The positions give different information to the doctor. As your chest moves while you breathe or pant, it changes the pressure and amount of air in the room and against the mouthpiece. From these changes, the doctor can get an accurate measure of the amount of air in your lungs.

Depending on the purpose of the test, you may be given medicine before the test.

How to Prepare for the Test

Let your doctor know if you are taking any medicines, especially ones for breathing problems. You may have to temporarily stop taking certain medicines before the test.

Wear loose clothes that allow you to breathe comfortably.

Avoid smoking and heavy exercise for 6 hours before the test.

Avoid heavy meals before the test. They can affect your ability to take deep breaths.

How the Test will Feel

The test involves rapid and normal breathing, and should not be painful. You may feel short of breath or lightheaded. You will be monitored at all times by a technician.

The mouthpiece may feel uncomfortable against your mouth.

If you have trouble in tight spaces, the box might make you anxious. But it is clear and you can see outside at all times.

Why the Test is Performed

The test is done to see how much air you can hold in your lungs during rest. It helps your doctor determine if a lung problem is due to damage to the lung structure, or a loss of the lungs' ability to expand (get bigger as air flows in).

Although this test is the most accurate way to measure how much air you can hold in your lungs, it is not often used because of its technical difficulties.

Normal Results

What Abnormal Results Mean

Abnormal results point to a problem in the lungs. This problem can be due to a breakdown of the lung structure, a problem with the chest wall and its muscles, or a problem with the lungs being able to expand and contract.

Lung plethysmography will not find the cause of the problem. But it helps the doctor narrow down the list of possible problems.